Method and apparatus for sidelink signaling radio bearer (srb) establishment in a wireless communication system

ABSTRACT

A method and apparatus are disclosed. A first sidelink Signaling Radio Bearer (SRB) for PC5-S message transmission is established. The first sidelink SRB is associated with a default Destination Layer-2 Identity (ID). A first PC5-S message is transmitted on the first sidelink SRB with a Source Layer-2 ID derived from a Layer-2 ID of the first UE and a Destination Layer-2 ID derived from the default Destination Layer-2 ID. A second PC5-S message is received from a second UE. The second PC5-S message is transmitted with a Source Layer-2 ID derived from a Layer-2 ID of the second UE and a Destination Layer-2 ID derived from the Layer-2 ID of the first UE. A second sidelink SRB for PC5-S message reception and/or transmission is established. The second sidelink SRB is associated with the Layer-2 ID of the second UE.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 17/097,681, filed Nov. 13, 2020, which claims priority to andthe benefit of U.S. Provisional Patent Application Ser. No. 62/940,460,filed Nov. 26, 2019; with the entire disclosures of each referencedpatent application fully incorporated herein by reference.

FIELD

This disclosure generally relates to wireless communication networks,and more particularly, to a method and apparatus for sidelink SRBestablishment in a wireless communication system.

BACKGROUND

With the rapid rise in demand for communication of large amounts of datato and from mobile communication devices, traditional mobile voicecommunication networks are evolving into networks that communicate withInternet Protocol (IP) data packets. Such IP data packet communicationcan provide users of mobile communication devices with voice over IP,multimedia, multicast and on-demand communication services.

An exemplary network structure is an Evolved Universal Terrestrial RadioAccess Network (E-UTRAN). The E-UTRAN system can provide high datathroughput in order to realize the above-noted voice over IP andmultimedia services. A new radio technology for the next generation(e.g., 5G) is currently being discussed by the 3GPP standardsorganization. Accordingly, changes to the current body of 3GPP standardare currently being submitted and considered to evolve and finalize the3GPP standard.

SUMMARY

A method and apparatus are disclosed from the perspective of a firstUser Equipment (UE) for establishing a unicast link. In one embodiment,the method includes the first UE establishing a first sidelink SignalingRadio Bearer (SRB) for PC5-S message transmission if a unicast link or aPC5-Radio Resource Control (PC5-RRC) connection establishment isinitiated, wherein the first sidelink SRB is associated with a defaultDestination Layer-2 Identity (ID). The method also includes the first UEtransmitting a first PC5-S message for the unicast link or the PC5-RRCconnection establishment on the first sidelink SRB with a Source Layer-2ID derived from a Layer-2 ID of the first UE and a Destination Layer-2ID derived from the default Destination Layer-2 ID. The method furtherincludes the first UE receiving a second PC5-S message from a second UE,wherein the second PC5-S message is transmitted with a Source Layer-2 IDderived from a Layer-2 ID of the second UE and a Destination Layer-2 IDderived from the Layer-2 ID of the first UE. In addition, the methodincludes the first UE establishing a second sidelink SRB for PC5-Smessage reception and/or transmission, wherein the second sidelink SRBis associated with the Layer-2 ID of the second UE. Furthermore, themethod includes the first UE establishing a third sidelink SRB forPC5-RRC message transmission if the unicast link or the PC5-RRCconnection establishment has been successfully completed, wherein thethird sidelink SRB is associated with the Layer-2 ID of the second UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a wireless communication system according toone exemplary embodiment.

FIG. 2 is a block diagram of a transmitter system (also known as accessnetwork) and a receiver system (also known as user equipment or UE)according to one exemplary embodiment.

FIG. 3 is a functional block diagram of a communication system accordingto one exemplary embodiment.

FIG. 4 is a functional block diagram of the program code of FIG. 3according to one exemplary embodiment.

FIG. 5 is a reproduction of FIG. 5.2.1.4-1 of 3GPP TS 23.287 V16.0.0.

FIG. 6 is a reproduction of FIG. 6.3.3.1-1 of 3GPP TS 23.287 V16.0.0.

FIG. 7 is a reproduction of FIG. 6.5.5.2-1 of 3GPP TS 33.303 V15.0.0.

FIG. 8 is a reproduction of FIG. 5.X.3.1-1 of 3GPP R2-1915983.

FIG. 9 is a reproduction of FIG. 6.1.x-1 of 3GPP R2-1916120.

FIG. 10 is a reproduction of FIG. 6.1.x-2 of 3GPP R2-1916120.

FIG. 11 is a diagram according to one exemplary embodiment.

FIG. 12 is a flow chart according to one exemplary embodiment.

FIG. 13 is a flow chart according to one exemplary embodiment.

FIG. 14 is a flow chart according to one exemplary embodiment.

DETAILED DESCRIPTION

The exemplary wireless communication systems and devices described belowemploy a wireless communication system, supporting a broadcast service.Wireless communication systems are widely deployed to provide varioustypes of communication such as voice, data, and so on. These systems maybe based on code division multiple access (CDMA), time division multipleaccess (TDMA), orthogonal frequency division multiple access (OFDMA),3GPP LTE (Long Term Evolution) wireless access, 3GPP LTE-A orLTE-Advanced (Long Term Evolution Advanced), 3GPP2 UMB (Ultra MobileBroadband), WiMax, 3GPP NR (New Radio), or some other modulationtechniques.

In particular, the exemplary wireless communication systems devicesdescribed below may be designed to support one or more standards such asthe standard offered by a consortium named “3rd Generation PartnershipProject” referred to herein as 3GPP, including: TS 23.287 V16.0.0,“Architecture enhancements for 5G System (5GS) to supportVehicle-to-Everything (V2X) services (Release 16)”; TS 33.303 V15.0.0,“Proximity-based Services (ProSe); Security aspects (Release 15);R2-1915983, “Running CR to TS 38.331 for 5G V2X with NR Sidelink”,Huawei, HiSilicon; R2-1916120, “Running CR to TS 38.321 for 5G V2X withNR Sidelink”, LG Electronics; and R2-1916288, “Report from session onLTE V2X and NR V2X”, Samsung. The standards and documents listed aboveare hereby expressly incorporated by reference in their entirety.

FIG. 1 shows a multiple access wireless communication system accordingto one embodiment of the invention. An access network 100 (AN) includesmultiple antenna groups, one including 104 and 106, another including108 and 110, and an additional including 112 and 114. In FIG. 1, onlytwo antennas are shown for each antenna group, however, more or fewerantennas may be utilized for each antenna group. Access terminal 116(AT) is in communication with antennas 112 and 114, where antennas 112and 114 transmit information to access terminal 116 over forward link120 and receive information from access terminal 116 over reverse link118. Access terminal (AT) 122 is in communication with antennas 106 and108, where antennas 106 and 108 transmit information to access terminal(AT) 122 over forward link 126 and receive information from accessterminal (AT) 122 over reverse link 124. In a FDD system, communicationlinks 118, 120, 124 and 126 may use different frequency forcommunication. For example, forward link 120 may use a differentfrequency then that used by reverse link 118.

Each group of antennas and/or the area in which they are designed tocommunicate is often referred to as a sector of the access network. Inthe embodiment, antenna groups each are designed to communicate toaccess terminals in a sector of the areas covered by access network 100.

In communication over forward links 120 and 126, the transmittingantennas of access network 100 may utilize beamforming in order toimprove the signal-to-noise ratio of forward links for the differentaccess terminals 116 and 122. Also, an access network using beamformingto transmit to access terminals scattered randomly through its coveragecauses less interference to access terminals in neighboring cells thanan access network transmitting through a single antenna to all itsaccess terminals.

An access network (AN) may be a fixed station or base station used forcommunicating with the terminals and may also be referred to as anaccess point, a Node B, a base station, an enhanced base station, anevolved Node B (eNB), or some other terminology. An access terminal (AT)may also be called user equipment (UE), a wireless communication device,terminal, access terminal or some other terminology.

FIG. 2 is a simplified block diagram of an embodiment of a transmittersystem 210 (also known as the access network) and a receiver system 250(also known as access terminal (AT) or user equipment (UE)) in a MIMOsystem 200. At the transmitter system 210, traffic data for a number ofdata streams is provided from a data source 212 to a transmit (TX) dataprocessor 214.

In one embodiment, each data stream is transmitted over a respectivetransmit antenna. TX data processor 214 formats, codes, and interleavesthe traffic data for each data stream based on a particular codingscheme selected for that data stream to provide coded data.

The coded data for each data stream may be multiplexed with pilot datausing OFDM techniques. The pilot data is typically a known data patternthat is processed in a known manner and may be used at the receiversystem to estimate the channel response. The multiplexed pilot and codeddata for each data stream is then modulated (i.e., symbol mapped) basedon a particular modulation scheme (e.g., BPSK, QPSK, M-PSK, or M-QAM)selected for that data stream to provide modulation symbols. The datarate, coding, and modulation for each data stream may be determined byinstructions performed by processor 230.

The modulation symbols for all data streams are then provided to a TXMIMO processor 220, which may further process the modulation symbols(e.g., for OFDM). TX MIMO processor 220 then provides N_(T) modulationsymbol streams to N_(T) transmitters (TMTR) 222 a through 222 t. Incertain embodiments, TX MIMO processor 220 applies beamforming weightsto the symbols of the data streams and to the antenna from which thesymbol is being transmitted.

Each transmitter 222 receives and processes a respective symbol streamto provide one or more analog signals, and further conditions (e.g.,amplifies, filters, and upconverts) the analog signals to provide amodulated signal suitable for transmission over the MIMO channel. N_(T)modulated signals from transmitters 222 a through 222 t are thentransmitted from N_(T) antennas 224 a through 224 t, respectively.

At receiver system 250, the transmitted modulated signals are receivedby N_(R) antennas 252 a through 252 r and the received signal from eachantenna 252 is provided to a respective receiver (RCVR) 254 a through254 r. Each receiver 254 conditions (e.g., filters, amplifies, anddownconverts) a respective received signal, digitizes the conditionedsignal to provide samples, and further processes the samples to providea corresponding “received” symbol stream.

An RX data processor 260 then receives and processes the N_(R) receivedsymbol streams from N_(R) receivers 254 based on a particular receiverprocessing technique to provide N_(T) “detected” symbol streams. The RXdata processor 260 then demodulates, deinterleaves, and decodes eachdetected symbol stream to recover the traffic data for the data stream.The processing by RX data processor 260 is complementary to thatperformed by TX MIMO processor 220 and TX data processor 214 attransmitter system 210.

A processor 270 periodically determines which pre-coding matrix to use(discussed below). Processor 270 formulates a reverse link messagecomprising a matrix index portion and a rank value portion.

The reverse link message may comprise various types of informationregarding the communication link and/or the received data stream. Thereverse link message is then processed by a TX data processor 238, whichalso receives traffic data for a number of data streams from a datasource 236, modulated by a modulator 280, conditioned by transmitters254 a through 254 r, and transmitted back to transmitter system 210.

At transmitter system 210, the modulated signals from receiver system250 are received by antennas 224, conditioned by receivers 222,demodulated by a demodulator 240, and processed by a RX data processor242 to extract the reserve link message transmitted by the receiversystem 250. Processor 230 then determines which pre-coding matrix to usefor determining the beamforming weights then processes the extractedmessage.

Turning to FIG. 3, this figure shows an alternative simplifiedfunctional block diagram of a communication device according to oneembodiment of the invention. As shown in FIG. 3, the communicationdevice 300 in a wireless communication system can be utilized forrealizing the UEs (or ATs) 116 and 122 in FIG. 1 or the base station (orAN) 100 in FIG. 1, and the wireless communications system is preferablythe LTE or NR system. The communication device 300 may include an inputdevice 302, an output device 304, a control circuit 306, a centralprocessing unit (CPU) 308, a memory 310, a program code 312, and atransceiver 314. The control circuit 306 executes the program code 312in the memory 310 through the CPU 308, thereby controlling an operationof the communications device 300. The communications device 300 canreceive signals input by a user through the input device 302, such as akeyboard or keypad, and can output images and sounds through the outputdevice 304, such as a monitor or speakers. The transceiver 314 is usedto receive and transmit wireless signals, delivering received signals tothe control circuit 306, and outputting signals generated by the controlcircuit 306 wirelessly. The communication device 300 in a wirelesscommunication system can also be utilized for realizing the AN 100 inFIG. 1.

FIG. 4 is a simplified block diagram of the program code 312 shown inFIG. 3 in accordance with one embodiment of the invention. In thisembodiment, the program code 312 includes an application layer 400, aLayer 3 portion 402, and a Layer 2 portion 404, and is coupled to aLayer 1 portion 406. The Layer 3 portion 402 generally performs radioresource control. The Layer 2 portion 404 generally performs linkcontrol. The Layer 1 portion 406 generally performs physicalconnections.

3GPP TS 23.287 specifies Vehicle-to-Everything (V2X) communicationrelated to unicast mode as follows:

5.1.2 Authorization and Provisioning for V2X Communications Over PC5Reference Point 5.1.2.1 Policy/Parameter Provisioning

The following information for V2X communications over PC5 referencepoint is provisioned to the UE:

-   -   1) Authorization policy:        -   When the UE is “served by E-UTRA” or “served by NR”:            -   PLMNs in which the UE is authorized to perform V2X                communications over PC5 reference point when “served by                E-UTRA” or “served by NR”.            -   For each above PLMN:                -   RAT(s) over which the UE is authorized to perform                    V2X communications over PC5 reference point.            -   When the UE is “not served by E-UTRA” and “not served by                NR”:                -   Indicates whether the UE is authorized to perform                    V2X communications over PC5 reference point when                    “not served by E-UTRA” and “not served by NR”.                -   RAT(s) over which the UE is authorized to perform                    V2X communications over PC5 reference point.    -   2) Radio parameters when the UE is “not served by E-UTRA” and        “not served by NR”:        -   Includes the radio parameters per PC5 RAT (i.e. LTE PC5, NR            PC5) with Geographical Area(s) and an indication of whether            they are “operator managed” or “non-operator managed”. The            UE uses the radio parameters to perform V2X communications            over PC5 reference point when “not served by E-UTRA” and            “not served by NR” only if the UE can reliably locate itself            in the corresponding Geographical Area. Otherwise, the UE is            not authorized to transmit.    -   Editor's note: The radio parameters (e.g. frequency bands) are        to be defined by RAN WGs. The reference to RAN specification        will be added when defined in RAN WGs.    -   NOTE 1: Whether a frequency band is “operator managed” or        “non-operator managed” in a given Geographical Area is defined        by local regulations.    -   3) Policy/parameters per RAT for PC5 Tx Profile selection:        -   The mapping of service types (e.g. PSIDs or ITS-AIDS) to Tx            Profiles.    -   Editor's note: The Tx Profiles are to be defined by RAN WGs. The        reference to RAN specification will be added when defined in RAN        WGs.    -   4) Policy/parameters related to privacy:        -   The list of V2X services, e.g. PSIDs or ITS-AIDS of the V2X            applications, with Geographical Area(s) that require privacy            support.    -   5) Policy/parameters when LTE PC5 is selected:        -   Same as specified in TS 23.285 [8] clause 4.4.1.1.2 item 3)            Policy/parameters except for the mapping of service types to            Tx Profiles and the list of V2X services with Geographical            Area(s) that require privacy support.    -   6) Policy/parameters when NR PC5 is selected:        -   The mapping of service types (e.g. PSIDs or ITS-AIDS) to V2X            frequencies with Geographical Area(s).        -   The mapping of Destination Layer-2 ID(s) and the V2X            services, e.g. PSIDs or ITS-AIDS of the V2X application for            broadcast.        -   The mapping of Destination Layer-2 ID(s) and the V2X            services, e.g. PSIDs or ITS-AIDS of the V2X application for            groupcast.        -   The mapping of default Destination Layer-2 ID(s) for initial            signalling to establish unicast connection and the V2X            services, e.g. PSIDs or ITS-AIDS of the V2X application.    -   NOTE 2: The same default Destination Layer-2 ID for unicast        initial signalling can be mapped to more than one V2X services.        In the case where different V2X services are mapped to distinct        default Destination Layer-2 IDs, when the UE intends to        establish a single unicast link that can be used for more than        one V2X services, the UE can select any of the default        Destination Layer-2 IDs to use for the initial signalling.        -   PC5 QoS mapping configuration:            -   Input from V2X application layer:                -   V2X service (e.g. PSID or ITS-AID).                -   (Optional) V2X Application Requirements for the V2X                    service, e.g. priority requirement, reliability                    requirement, delay requirement, range requirement.    -   NOTE 3: Details of V2X Application Requirements for the V2X        service is up to implementation and out of scope of this        specification.        -   Output:            -   PC5 QoS parameters defined in clause 5.4.2 (i.e. PQI and                conditionally other parameters such as MFBR/GFBR, etc).    -   SLRB configurations, i.e. the mapping of PC5 QoS profile(s) to        SLRB(s), when the UE is “not served by E-UTRA” and “not served        by NR”.        -   The PC5 QoS profile contains PC5 QoS parameters described in            clause 5.4.2, and value for the QoS characteristics            regarding Priority Level, Averaging Window, Maximum Data            Burst Volume if default value is not used as defined in            Table 5.4.4-1.    -   Editor's note: The SLRB configurations will be determined by RAN        WGs. The reference to RAN specification will be added when        defined in RAN WGs.    -   Editor's note: For the PC5 QoS profile, coordination with RAN        WGs is needed.    -   Editor's note: The V2X frequencies with Geographical Area(s)        will be determined by RAN WGs. The reference to RAN        specification will be added when defined in RAN WGs.

5.2.1.4 Unicast Mode Communication Over PC5 Reference Point

Unicast mode of communication is only supported over NR based PC5reference point. FIG. 5.2.1.4-1 illustrates an example of PC5 unicastlinks.

-   -   [FIG. 5.2.1.4-1 of 3GPP TS 23.287 V16.0.0, entitled “Example of        PC5 Unicast Links”, is reproduced as FIG. 5]

The following principles apply when the V2X communication is carriedover PC5 unicast link:

-   -   A PC5 unicast link between two UEs allows V2X communication        between one or more pairs of peer V2X services in these UEs. All        V2X services in the UE using the same PC5 unicast link use the        same Application Layer ID.    -   NOTE 1: An Application Layer ID may change in time as described        in clauses 5.6.1.1 and 6.3.3.2, due to privacy. This does not        cause a re-establishment of a PC5 unicast link.        -   One PC5 unicast link supports one or more V2X services (e.g.            PSIDs or ITS-AIDS) if these V2X services are at least            associated with the pair of peer Application Layer IDs for            this PC5 unicast link. For example, as illustrated in FIG.            5.2.1.4-1, UE A and UE B have two PC5 unicast links, one            between peer Application Layer ID 1/UE A and Application            Layer ID 2/UE B and one between peer Application Layer ID            3/UE A and Application Layer ID 4/UE B.    -   NOTE 2: A source UE is not required to know whether different        target Application Layer IDs over different PC5 unicast links        belong to the same target UE.        -   A PC5 unicast link supports V2X communication using a single            network layer protocol e.g. IP or non-IP.        -   A PC5 unicast link supports per-flow QoS model as specified            in clause 5.4.1.

When the Application layer in the UE initiates data transfer for a V2Xservice which requires unicast mode of communication over PC5 referencepoint:

-   -   the UE shall reuse an existing PC5 unicast link if the pair of        peer Application Layer IDs and the network layer protocol of        this PC5 unicast link are identical to those required by the        application layer in the UE for this V2X service, and modify the        existing PC5 unicast link to add this V2X service as specified        in clause 6.3.3.4; otherwise    -   the UE shall trigger the establishment of a new PC5 unicast link        as specified in clause 6.3.3.1.

After successful PC5 unicast link establishment, UE A and UE B use thesame pair of Layer-2 IDs for subsequent PC5-S signalling messageexchange and V2X service data transmission as specified in clause5.6.1.4. The V2X layer of the transmitting UE indicates to the AS layerwhether a transmission is for a PC5-S signalling message (i.e. DirectCommunication Request/Accept, Link Identifier Update Request/Response,Disconnect Request/Response, Link Modification Request/Accept) or V2Xservice data.

For every PC5 unicast link, a UE self-assigns a distinct PC5 LinkIdentifier that uniquely identifies the PC5 unicast link in the UE forthe lifetime of the PC5 unicast link. Each PC5 unicast link isassociated with a Unicast Link Profile which includes:

-   -   service type(s) (e.g. PSID or ITS-AID), Application Layer ID and        Layer-2 ID of UE A; and    -   Application Layer ID and Layer-2 ID of UE B; and    -   network layer protocol used on the PC5 unicast link; and    -   for each V2X service, a set of PC5 QoS Flow Identifier(s)        (PFI(s)). Each PFI is associated with QoS parameters (i.e. PQ1        and optionally Range).

For privacy reason, the Application Layer IDs and Layer-2 IDs may changeas described in clauses 5.6.1.1 and 6.3.3.2 during the lifetime of thePC5 unicast link and, if so, shall be updated in the Unicast LinkProfile accordingly. The UE uses PC5 Link Identifier to indicate the PC5unicast link to V2X Application layer, therefore V2X Application layeridentifies the corresponding PC5 unicast link even if there are morethan one unicast link associated with one service type (e.g. the UEestablishes multiple unicast links with multiple UEs for a same servicetype).

The Unicast Link Profile shall be updated accordingly after a Layer-2link modification for an established PC5 unicast link as specified inclause 6.3.3.4.

5.6 Identifiers 5.6.1 Identifiers for V2X Communication Over PC5Reference Point 5.6.1.1 General

Each UE has one or more Layer-2 IDs for V2X communication over PC5reference point, consisting of:

-   -   Source Layer-2 ID(s); and    -   Destination Layer-2 ID(s).

Source and destination Layer-2 IDs are included in layer-2 frames senton the layer-2 link of the PC5 reference point identifying the layer-2source and destination of these frames. Source Layer-2 IDs are alwaysself-assigned by the UE originating the corresponding layer-2 frames.

The selection of the source and destination Layer-2 ID(s) by a UEdepends on the communication mode of V2X communication over PC5reference point for this layer-2 link, as described in clauses 5.6.1.2,5.6.1.3, and 5.6.1.4. The source Layer-2 IDs may differ betweendifferent communication modes.

When IP-based V2X communication is supported, the UE configures a linklocal IPv6 address to be used as the source IP address, as defined inclause 4.5.3 of TS 23.303 [17]. The UE may use this IP address for V2Xcommunication over PC5 reference point without sending NeighbourSolicitation and Neighbour Advertisement message for Duplicate AddressDetection.

If the UE has an active V2X application that requires privacy support inthe current Geographical Area, as identified by configuration describedin clause 5.1.2.1, in order to ensure that a source UE (e.g. vehicle)cannot be tracked or identified by any other UEs (e.g. vehicles) beyonda certain short time-period required by the application, the sourceLayer-2 ID shall be changed over time and shall be randomized. ForIP-based V2X communication over PC5 reference point, the source IPaddress shall also be changed over time and shall be randomized. Thechange of the identifiers of a source UE must be synchronized acrosslayers used for PC5, e.g. when the Application Layer ID changes, thesource Layer-2 ID and the source IP address need to be changed.

5.6.1.2 Identifiers for Broadcast Mode V2X Communication Over PC5Reference Point

For broadcast mode of V2X communication over PC5 reference point, the UEis configured with the destination Layer-2 ID(s) to be used for V2Xservices. The destination Layer-2 ID for a V2X communication is selectedbased on the configuration as described in clause 5.1.2.1.

The UE self-selects a source Layer-2 ID. The UE may use different sourceLayer-2 IDs for different types of PC5 reference points, i.e. LTE basedPC5 and NR based PC5.

5.6.1.3 Identifiers for Groupcast Mode V2X Communication Over PC5Reference Point

For groupcast mode of V2X communication over PC5 reference point, theV2X application layer may provide group identifier information. When thegroup identifier information is provided by the V2X application layer,the UE converts the provided group identifier into a destination Layer-2ID. When the group identifier information is not provided by the V2Xapplication layer, the UE determines the destination Layer-2 ID based onconfiguration of the mapping between service type (e.g. PSID/ITS-AID)and Layer-2 ID, as specified in clause 5.1.2.1.

-   -   NOTE: The mechanism for converting the V2X application layer        provided group identifier to the destination Layer-2 ID is        defined in Stage 3.

The UE self-selects a source Layer-2 ID.

-   -   Editor's note: Further updates of the identifiers description        may be required based on RAN WG feedback.

5.6.1.4 Identifiers for Unicast Mode V2X Communication Over PC5Reference Point

For unicast mode of V2X communication over PC5 reference point, thedestination Layer-2 ID used depends on the communication peer, which isdiscovered during the establishment of the PC5 unicast link. The initialsignalling for the establishment of the PC5 unicast link may use adefault destination Layer-2 ID associated with the service type (e.g.PSID/ITS-AID) configured for PC5 unicast link establishment, asspecified in clause 5.1.2.1. During the PC5 unicast link establishmentprocedure, Layer-2 IDs are exchanged, and should be used for futurecommunication between the two UEs, as specified in clause 6.3.3.1.

The Application Layer ID is associated with one or more V2X applicationswithin the UE. If UE has more than one Application Layer IDs, eachApplication Layer ID of the same UE may be seen as different UE'sApplication Layer ID from the peer UE's perspective.

The UE maintains a mapping between the Application Layer IDs and thesource Layer-2 IDs used for the PC5 unicast links, as the V2Xapplication layer does not use the Layer-2 IDs. This allows the changeof source Layer-2 ID without interrupting the V2X applications.

When Application Layer IDs change, the source Layer-2 ID(s) of the PC5unicast link(s) shall be changed if the link(s) was used for V2Xcommunication with the changed Application Layer IDs.

A UE may establish multiple PC5 unicast links with a peer UE and use thesame or different source Layer-2 IDs for these PC5 unicast links.

-   -   Editor's note: Further updates of the identifier description may        be required based on RAN WG feedback.

6.3.3 Unicast Mode V2X Communication Over PC5 Reference Point 6.3.3.1Layer-2 Link Establishment Over PC5 Reference Point

To perform unicast mode of V2X communication over PC5 reference point,the UE is configured with the related information as described in clause5.1.2.1.

FIG. 6.3.3.1-1 shows the layer-2 link establishment procedure forunicast mode of V2X communication over PC5 reference point.

-   -   [FIG. 6.3.3.1-1 of 3GPP TS 23.287 V16.0.0, entitled “Layer-2        link establishment procedure”, is reproduced as FIG. 6]        -   1. The UE(s) determine the destination Layer-2 ID for            signalling reception for PC5 unicast link establishment as            specified in clause 5.6.1.4. The destination Layer-2 ID is            configured with the UE(s) as specified in clause 5.1.2.1.        -   2. The V2X application layer in UE-1 provides application            information for PC5 unicast communication. The application            information includes the service type(s) (e.g. PSID or            ITS-AID) of the V2X application and the initiating UE's            Application Layer ID. The target UE's Application Layer ID            may be included in the application information.            -   The V2X application layer in UE-1 may provide V2X                Application Requirements for this unicast communication.                UE-1 determines the PC5 QoS parameters and PFI as                specified in clause 5.4.1.4.            -   If UE-1 decides to reuse the existing PC5 unicast link                as specified in clause 5.2.1.4, the UE triggers Layer-2                link modification procedure as specified in clause                6.3.3.4.        -   3. UE-1 sends a Direct Communication Request message to            initiate the unicast layer-2 link establishment procedure.            The Direct Communication Request message includes:            -   Source User Info: the initiating UE's Application Layer                ID (i.e. UE-1's Application Layer ID).            -   If the V2X application layer provided the target UE's                Application Layer ID in step 2, the following                information is included:                -   Target User Info: the target UE's Application Layer                    ID (i.e. UE-2's Application Layer ID).            -   V2X Service Info: the information about V2X Service(s)                requesting Layer-2 link establishment (e.g. PSID(s) or                ITS-AID(s)).            -   Indication whether IP communication is used.            -   IP Address Configuration: For IP communication, IP                address configuration is required for this link and                indicates one of the following values:                -   “IPv6 Router” if IPv6 address allocation mechanism                    is supported by the initiating UE, i.e., acting as                    an IPv6 Router; or                -   “IPv6 address allocation not supported” if IPv6                    address allocation mechanism is not supported by the                    initiating UE.            -   Link Local IPv6 Address: a link-local IPv6 address                formed locally based on RFC 4862 [21] if UE-1 does not                support the IPv6 IP address allocation mechanism, i.e.                the IP Address Configuration indicates “IPv6 address                allocation not supported”.            -   QoS Info: the information about PC5 QoS Flow(s). For                each PC5 QoS Flow, the PFI and the corresponding PC5 QoS                parameters (i.e. PQI and conditionally other parameters                such as MFBR/GFBR, etc).        -   The source Layer-2 ID and destination Layer-2 ID used to            send the Direct Communication Request message are determined            as specified in clauses 5.6.1.1 and 5.6.1.4.        -   UE-1 sends the Direct Communication Request message via PC5            broadcast using the source Layer-2 ID and the destination            Layer-2 ID.        -   4. A Direct Communication Accept message is sent to UE-1 as            below:            -   4a. (UE oriented Layer-2 link establishment) If the                Target User Info is included in the Direct Communication                Request message, the target UE, i.e. UE-2 responds with                a Direct Communication Accept message.            -   4b. (V2X Service oriented Layer-2 link establishment) If                the Target User Info is not included in the Direct                Communication Request message, the UEs that are                interested in using the announced V2X Service(s), so                decide to establish Layer-2 link with UE-1 respond to                the request by sending a Direct Communication Accept                message (UE-2 and UE-4 in FIG. 6.3.3.1-1).            -   The Direct Communication Accept message includes:                -   Source User Info: Application Layer ID of the UE                    sending the Direct Communication Accept message.                -   QoS Info: the information about PC5 QoS Flow(s). For                    each PC5 QoS Flow, the PFI and the corresponding PC5                    QoS parameters requested by UE-1 (i.e. PQI and                    conditionally other parameters such as MFBR/GFBR,                    etc).                -   IP Address Configuration: For IP communication, IP                    address configuration is required for this link and                    indicates one of the following values:                -    “IPv6 Router” if IPv6 address allocation mechanism                    is supported by the target UE, i.e., acting as an                    IPv6 Router; or                -    “IPv6 address allocation not supported” if IPv6                    address allocation mechanism is not supported by the                    target UE.                -   Link Local IPv6 Address: a link-local IPv6 address                    formed locally based on RFC 4862 [21] if the target                    UE does not support the IPv6 IP address allocation                    mechanism, i.e. the IP Address Configuration                    indicates “IPv6 address allocation not supported”,                    and UE-1 included a link-local IPv6 address in the                    Direct Communication Request message. The target UE                    shall include a non-conflicting link-local IPv6                    address.            -   If both UEs (i.e. the initiating UE and the target UE)                selected to use link-local IPv6 address, they shall                disable the duplicate address detection defined in RFC                4862 [21].        -   NOTE 1: When either the initiating UE or the target UE            indicates the support of IPv6 router, corresponding address            configuration procedure would be carried out after the            establishment of the layer 2 link, and the link-local IPv6            addresses are ignored.            -   The source Layer-2 ID used to send the Direct                Communication Accept message is determined as specified                in clauses 5.6.1.1 and 5.6.1.4. The destination Layer-2                ID is set to the source Layer-2 ID of the received                Direct Communication Request message.            -   Upon receiving the Direct Communication Accept message                from peer UE, UE-1 obtains the peer UE's Layer-2 ID for                future communication, for signalling and data traffic                for this unicast link.            -   The V2X layer of the UE that established PC5 unicast                link passes the PC5 Link Identifier assigned for the                unicast link and PC5 unicast link related information                down to the AS layer. The PC5 unicast link related                information includes Layer-2 ID information (i.e. source                Layer-2 ID and destination Layer-2 ID). This enables the                AS layer to maintain the PC5 Link Identifier together                with the PC5 unicast link related information.        -   Editor's note: Steps for mutual authentication and security            association establishment will be determined based on            feedback from SA WG3.        -   5. V2X service data is transmitted over the established            unicast link as below:            -   The PC5 Link Identifier and PFI are provided to the AS                layer, together with the V2X service data.            -   UE-1 sends the V2X service data using the source Layer-2                ID (i.e. UE-Vs Layer-2 ID for this unicast link) and the                destination Layer-2 ID (i.e. the peer UE's Layer-2 ID                for this unicast link).        -   NOTE 2: PC5 unicast link is bi-directional, therefore the            peer UE of UE-1 can send the V2X service data to UE-1 over            the unicast link with UE-1.        -   Editor's note: The parameters included in the Direct            Communication Request/Accept messages can be updated            depending on RAN WGs' decision on how the Direct            Communication Request/Accept messages are sent by the AS            layer (e.g. by using PC5-RRC signalling).        -   Editor's note: Additional parameters included in the Direct            Communication Request/Accept messages (e.g. security            related) are FFS.        -   Editor's note: Whether the unicast communication requires            security protection at link layer will be determined based            on feedback from SA WG3.

In 3GPP TS 33.303, security for direct link signaling and direct linkuser plane traffic in one-to-one sidelink communication (i.e. unicastmode communication) is specified as follows:

6.5 Security for One-to-One ProSe Direct Communication 6.5.2 SecurityRequirements

The following are the security requirements for ProSe Direct One-to-oneCommunication:

-   -   A ProSe-enabled UE shall use different security contexts for        ProSe one-to-one communication with different ProSe-enabled UEs.    -   Direct link signalling ciphering shall be supported and may be        used. Direct link signalling ciphering is a configuration        option.    -   Direct link user plane ciphering shall be supported and may be        used.    -   Direct link signalling integrity protection and replay        protection shall be supported and used.    -   Direct link user plane packets between UEs shall not be        integrity protected.    -   Establishment of the security between the UEs shall be protected        from man-in-the-middle attacks.    -   The system should support mutual authentication of public safety        UEs out of network coverage.    -   Compromise of a single UE should not affect the security of the        others.    -   Authentication credentials should be securely stored in UE.

6.5.3 Overview of One-to-One ProSe Direct Communication

6.5.3.1 Description of Different Layers of Keys and their Identities

ProSe Direct One-to-one communication uses 4 different layers of keys.These are the following:

-   -   Long term key: This is the key that is provisioned (see the        individual cases in 6.5.4 for more information on the        provisioning) into the UE and is the root of the security for        one-to-one communications. It may be a symmetric key or        public/private key pair depending on the particular use case.        Authentication signalling (denoted as “Direct Authentication and        Key Establishment”—see subclause 6.5.4) is exchanged between the        UEs and possibly some entities in the network, for example in        the ProSe UE-to-network relay case to derive the K_(D). The long        term key is identified by the Long term ID.    -   K_(D): This is a 256-bit root key that is shared between the two        entities communicating using ProSe Direct one-to-one        communications. It may be refreshed by re-running the        authentication signalling using the Long term key. In order to        generate a K_(D-sess) (the next layer of keys), nonces are        exchanged between the communicating entities. K_(D) may be kept        even when the UEs have no active one-to-one communication        session between them. The K_(D) ID is used to identify K_(D).    -   K_(D-sess): This is the 256-bit key that is the root of the        actual security context that is being used (or at least in the        process of being established) to protect the transfer of data        between the UEs. During a communication between the UEs, the        K_(D-sess) may be refreshed by running the rekeying procedure        (see subclause 6.X.5.3). The actual keys (see next bullet) that        are used in the confidentiality and integrity algorithms are        derived directly from K_(D-sess). The 16 bit K_(D-sess) ID        identifies the K_(D-Sess).    -   A K_(D-sess) ID with a zero value indicates no security is used        and hence the UEs shall not assign an all zero value of        K_(D-sess) ID when creating a security context.    -   PEK and PIK: The ProSe Encryption Key (PEK) and ProSe Integrity        Key (PIK) are used in the chosen confidentiality and integrity        algorithms respectively. They are derived from K_(D-sess) and        are refreshed automatically every time K_(D-sess) is changed.

6.5.5.2 Security Establishment During Connection Set-Up

The subclause describes how security is established during connectionset-up. The signalling flow is shown in FIG. 6.5.5.2-1.

-   -   [FIG. 6.5.5.2-1 of 3GPP TS 33.303 V15.0.0, entitled “Security        establishment at connection set-up”, is reproduced as FIG. 7]        -   1. UE_1 has sent a Direct Communication Request to UE_2.            This message shall include Nonce_1 (for session key            generation), UE_1 security capabilities (the list of            algorithms that UE_1 will accept for this connection) and            the most significant 8-bits of the K_(D-sess ID). These bits            shall be chosen such that UE_1 will be able to locally            identify a security context that is created by this            procedure. The message may also include a K_(D) ID if the            UE_1 has an existing K_(D) with the UE that it trying to            communicate with. The absence of the K_(D) ID parameter            indicates that UE_1 does not have a K_(D) for UE_2. The            message shall also contain the necessary information to            establish a K_(D) from the relevant long terms keys held on            the UE (see subclause 6.X.4). Long term ID is the info            needed by the UE_2 in order to retrieve the right Long term            Key.        -   2. UE_2 may initiate a Direct Auth and Key Establish            procedure with UE_1. This is mandatory if the UE_2 does not            have the K_(D) and K_(D) ID pair indicated in step 1, and            signalling is needed to establish the keys for the            particular use case.        -   3. UE_2 sends the Direct Security Mode Command to UE_1. It            shall include the most significant bits of K_(D) ID if a            fresh K_(D) is generated, Nonce_2 to allow a session key to            be calculated and the Chosen_algs parameter to indicate            which security algorithms the UEs will use to protect the            data. The included bits of K_(D) ID shall uniquely identify            the K_(D) at UE_2. UE_2 shall also return the UE_1 security            capabilities to provide protection against bidding down            attacks. UE_2 also includes the least significant 8-bits of            K_(D-sess) ID in the messages. This bits are chosen so that            UE_2 will be able to locally identify a security context            that is created by this procedure. UE_2 calculates            K_(D-sess) from K_(D) and Nonce_1 and Nonce_2 (see Annex            A.9) and then derives the confidentiality and integrity keys            based on the chosen algorithms (Annex A.4).            -   UE_2 then integrity protects the Direct Security Mode                Command before sending it to UE_1. UE_2 is then ready to                receive both signalling and user plane traffic protected                with the new security context. UE_2 shall form the                K_(D-sess) ID from the most significant bits it received                in message 1 and least significant bits it sent in                message 3.        -   4. On receiving the Direct Security Mode Command, UE_1 shall            calculate K_(D-sess) and the confidentiality and integrity            keys in the same way as UE_2. UE_1 shall check that the            returned UE_1 security capabilities are the same as those it            sent in step 1. UE_1 shall also check the integrity            protection on the message. If both these checks pass, then            UE_1 is ready to send and receive signalling and user            traffic with the new security context. If most significant            bits of K_(D) ID were included in the Direct Security Mode            Command, UE_1 shall generate the least significant bits of            K_(D) ID such that these bits uniquely identify K_(D) at            UE_1 and shall store the complete K_(D) ID with K_(D). UE_1            shall send an integrity protected and confidentiality            protected (with the chosen algorithm which may be the null            algorithm) Direct Security Mode Complete message to UE_2.            UE_1 shall include the least significant bits of K_(D) ID in            this message. UE_1 shall form the K_(D-sess) ID from the            most significant bits it sent in message 1 and least            significant bits it received in message 3.        -   5. UE_2 checks the integrity protection on the received            Direct Security Mode Complete. If this passes, UE_2 is now            ready to send user plane data and control signalling            protected with the new security context. UE_2 deletes any            old security context it has for UE_1. UE_2 shall form the            K_(D) ID from the most significant bits it sent in step 3            and least significant bits it received in the Direct            Security Mode Complete.UE_2 shall store the complete K_(D)            ID with K_(D).

A running CR to 3GPP TS 38.331 for 5G V2X with NR Sidelink (as capturedin 3GPP R2-1915983) specifies procedures related to NR sidelinkcommunication as follows:

5.X.3 Sidelink UE Information for NR Sidelink Communication 5.X.3.1General

-   -   [FIG. 5.X.3.1-1 of 3GPP R2-1915983, entitled “Sidelink UE        information for NR sidelink communication”, is reproduced as        FIG. 8]

The purpose of this procedure is to inform the network that the UE isinterested or no longer interested to receive NR sidelink communication,as well as to request assignment or release of transmission resource forNR sidelink communication and to report parameters related to NRsidelink communication.

5.x.3.2 Initiation

A UE capable of NR sidelink communication that is in RRC_CONNECTED mayinitiate the procedure to indicate it is (interested in) receiving NRsidelink communication in several cases including upon successfulconnection establishment or resuming, upon change of interest, uponchange to a PCell broadcasting SIBX including sl-ConfigCommonNR. A UEcapable of NR sidelink communication may initiate the procedure torequest assignment of dedicated resources for NR sidelink communicationtransmission.

Upon initiating this procedure, the UE shall:

-   -   1> if SIBX including sl-ConfigCommonNR is broadcast by the        PCell:        -   2> ensure having a valid version of SIBX for the PCell;        -   2> if configured by upper layers to receive NR sidelink            communication on the frequency included in sl-FreqInfoList            in SIBX of the PCell:            -   3> if the UE did not transmit a SidelinkUEInformationNR                message since last entering RRC_CONNECTED state; or            -   3> if since the last time the UE transmitted a                SidelinkUEInformationNR message the UE connected to a                PCell not broadcasting SIBX including sl-ConfigCommonNR;                or            -   3> if the last transmission of the                SidelinkUEInformationNR message did not include                sl-RxInterestedFreqList; or if the frequency configured                by upper layers to receive NR sidelink communication on                has changed since the last transmission of the                SidelinkUEInformationNR message:                -   4> initiate transmission of the                    SidelinkUEInformationNR message to indicate the NR                    sidelink communication reception frequency of                    interest in accordance with 5.x.3.3,        -   2> else:            -   3> if the last transmission of the                SidelinkUEInformationNR message included                sl-RxInterestedFreqList:                -   4> initiate transmission of the                    SidelinkUEInformationNR message to indicate it is no                    longer interested in NR sidelink communication                    reception in accordance with 5.x.3.3;        -   2> if configured by upper layers to transmit NR sidelink            communication on the frequency included in sl-FreqInfoList            in SIBX of the PCell:            -   3> if the UE did not transmit a SidelinkUEInformationNR                message since last entering RRC_CONNECTED state; or            -   3> if since the last time the UE transmitted a                SidelinkUEInformationNR message the UE connected to a                PCell not broadcasting SIBX including sl-ConfigCommonNR;                or            -   3> if the last transmission of the                SidelinkUEInformationNR message did not include                sl-TxResourceReqList; or if the information carried by                the sl-TxResourceReqList has changed since the last                transmission of the SidelinkUEInformationNR message:                -   4> initiate transmission of the                    SidelinkUEInformationNR message to indicate the NR                    sidelink communication transmission resources                    required by the UE in accordance with 5.X.3.3;        -   2> else:            -   3> if the last transmission of the                SidelinkUEInformationNR message included                sl-TxResourceReqList:                -   4> initiate transmission of the                    SidelinkUEInformationNR message to indicate it no                    longer requires NR sidelink communication                    transmission resources in accordance with 5.X.3.3.

5.x.3.3 Actions related to transmission of SidelinkUEInformationNRmessage

The UE shall set the contents of the SidelinkUEInformationNR message asfollows:

-   -   1> if the UE initiates the procedure to indicate it is (no more)        interested to receive NR sidelink communication or to request        (configuration/release) of NR sidelink communication        transmission resources (i.e. UE includes all concerned        information, irrespective of what triggered the procedure):        -   2> if SIBX including sl-ConfigCommonNR is broadcast by the            PCell:            -   3> if configured by upper layers to receive NR sidelink                communication:                -   4> include sl-RxInterestedFreqList and set it to the                    frequency for NR sidelink communication reception;            -   3> if configured by upper layers to transmit NR sidelink                communication:                -   4> include sl-TxResourceReqList and set its fields                    as follows for each destination for which it                    requests network to assign NR sidelink communication                    resource:                -    5> set sl-DestinationIdentiy to the destination                    identity configured by upper layer for NR sidelink                    communication transmission;                -    5> set sl-CastType to the cast type of the                    associated destination identity configured by the                    upper layer for the NR sidelink communication                    transmission;                -    5> set sl-QoS-InfoList to include QoS profile(s) of                    the sidelink QoS flow(s) of the associated                    destination configured by the upper layer for the NR                    sidelink communication transmission;                -    5> set sl-InterestedFreqList to indicate the                    frequency for NR sidelink communication                    transmission;                -    5> set sl-TypeTxSyncList to the current                    synchronization reference type used on the                    associated sl-InterestedFreqList for NR sidelink                    communication transmission.    -   1> The UE shall submit the SidelinkUEInformationNR message to        lower layers for transmission.    -   Editor's Notes: FFS on how to handle the RLF case with or        without failure indication.

[ . . . ]

5.x.9.1.6 Sidelink SRB Addition

The UE shall:

-   -   1> if a PC5-RRC connection establishment for a specific        destination is requested by upper layers:        -   2> establish PDCP entity, RLC entity and the logical channel            of a sidelink SRB for PC5-S message, as specified in            sub-clause 9.1.1.X;        -   2> establish PDCP entity, RLC entity and the logical channel            of a sidelink SRB for PC5-RRC message, as specified in            sub-clause 9.1.1.X;        -   2> consider the PC5-RRC connection is established for the            destination.

5.x.9.1.7 Sidelink SRB Release

The UE shall:

-   -   1> if a PC5-RRC connection release for a specific destination is        requested by upper layers; or    -   1> if the sidelink radio link failure is detected for a specific        destination:        -   2> release the PDCP entity, RLC entity and the logical            channel of the sidelink SRB for PC5-RRC message of the            destination;        -   2> consider the PC5-RRC connection is released for the            destination.    -   1> if a PC5-S connection release for a specific destination is        requested by upper layers; or        -   2> release the PDCP entity, RLC entity and the logical            channel of the sidelink SRB for PC5-S message of the            destination;

[ . . . ]

9.1.1.X SCCH Configuration

Parameters that are specified for unicast of NR sidelink communication,which is used for the sidelink signalling radio bearer of PC5-RRCmessage.

Name Value Semantics description Ver PDCP configuration >t-ReorderingUndefined Selected by the receiving UE, up to UEimplementation >pdcp-SN-Size 12 (FFS) RLC configuration >sn-FieldLength12 >t-Reassembly Undefined Selected by the receiving UE, up to Up to UEimplementation >logicalChannelIdentity 1 MAC configuration >priority1 >prioritisedBitRate infinity >logicalChannelGroup 0

Parameters that are specified for unicast of NR sidelink communication,which is used for the sidelink signalling radio bearer of PC5-S message.

Name Value Semantics description Ver PDCP configuration >t-ReorderingUndefined Selected by the receiving UE, up to UEimplementation >pdcp-SN-Size 12 RLC configuration >sn-FieldLength12 >t-Reassembly Undefined Selected by the receiving UE, up to Up to UEimplementation >logicalChannelIdentity 0 MAC configuration >priority1 >prioritisedBitRate infinity >logicalChannelGroup 0

A running CR to 3GPP TS 38.321 for 5G V2X with NR Sidelink (as capturedin 3GPP R2-196120) specifies MAC PDU for NR sidelink communication asfollows:

6.1.x MAC PDU (SL-SCH)

A MAC PDU consists of one SL-SCH subheader and one or more MAC subPDUs.Each MAC subPDU consists of one of the following:

-   -   A MAC subheader only (including padding);    -   A MAC subheader and a MAC SDU;    -   A MAC subheader and padding.

The MAC SDUs are of variable sizes.

Each MAC subheader except SL-SCH subheader corresponds to either a MACSDU or padding.

The SL-SCH subheader is of a fixed size and consists of the seven headerfields [V/R/R/R/R/SRC/DST].

-   -   [FIG. 6.1.x-1 of 3GPP R2-1916120, entitled “SL-SCH MAC        subheader”, is reproduced as FIG. 9]

Editor's Note: FFS on the format of the SL-SCH MAC subheader e.g. needof V field, SRC/DST sizes.

A MAC subheader except for padding consists of the four header fieldsR/F/LCID/L as depicted in FIG. 6.1.2-1 (with 8-bit L field) and FIG.6.1.2-2 (with 16-bit L field). A MAC subheader for padding consists ofthe two header fields R/LCID as depicted in FIG. 6.1.2-3.

SL MAC subPDU(s) with MAC SDU(s) is placed after the SL-SCH subheaderand before the MAC subPDU with padding in the MAC PDU as depicted inFIG. 6.1.x-1. The size of padding can be zero.

-   -   [FIG. 6.1.x-2 of 3GPP R2-1916120, entitled “Example of a SL MAC        PDU”, is reproduced as FIG. 10]

A maximum of one MAC PDU can be transmitted per TB per MAC entity.

The following agreements on Source ID and Destination ID indication forNR sidelink transmission were made in the RAN2 #108 meeting (as capturedin 3GPP R2-1916288):

-   1: For all cast-types, Layer-1 Destination ID corresponds to the 16    bit LSB of the Destination Layer-2 ID, and the Layer-1 Source ID    corresponds to the 8 bit LSB of the Source Layer-2 ID.-   2: The DST field includes 8 bit MSB of the Destination Layer-2 ID    and the SRC field includes 16 bit MSB of the Source Layer-2 ID for    the SL-SCH subheader of a MAC PDU to be transmitted to the peer UE.

3GPP TS 23.287 specifies a layer-2 link establishment procedure forunicast mode of V2X communication over PC5 reference point in Section6.3.3.1. For example, the initiating UE (e.g. UE1) transmits a DirectCommunication Request message and receives a Direct Communication Acceptmessage from other UE(s). According to Section 5.6.1.4 of 3GPP TS23.287, the initial signalling for the establishment of the PC5 unicastlink may use a default destination Layer-2 ID associated with theservice type (e.g. PSID/ITS-AID) configured for PC5 unicast linkestablishment.

In the Direct Communication Request message, UE2's application layer IDand UE1's application layer ID are included so that UE2 can determine ifto respond to the Direct Communication Request message. If UE2determines to respond to the Direct Communication Request message, UE2may initialize the procedure used to establish the security context. Asecurity configuration procedure used to establish security contextduring sidelink connection setup or unicast link establishment wasspecified in 3GPP TS 33.303. For example, UE1 transmits a DirectCommunication Request to UE2. In the Direct Communication Request, someparameters used to establish security context could be included. Uponreception of the Direct Communication Request, UE2 may optionallyinitiate a Direct Auth and Key Establish procedure with UE1 to establisha security key if the security key is not present. And then, UE2 mayactivate the security functionality. For example, UE2 may transmit aDirect Security Mode Command to UE1, and UE1 responds to UE2 with aDirect Security Mode Complete. In addition, if the Direct Security ModeComplete is received successfully, UE2 may transmit a DirectCommunication Accept to UE1. In case security is not needed for aunicast link, the related security procedures may be omitted, and UE2may reply the Direct Communication Accept to UE1 directly.

When the Direct Communication Request message is transmitted, the sourceLayer-2 ID is derived from (or set to) Layer-2 ID of the initiating UEand the destination Layer-2 ID is derived from (or set to) the defaultdestination Layer-2 ID associated with the service type. Then, UE2 maystart to exchange signalling in the security establishment procedurebased on the Layer-2 ID of UE1 and a Layer-2 ID of UE2 assigned for theunicast link after receiving the Direct Communication Request message.

More specifically, a Layer-2 ID may be split into two parts, one part isindicated in Layer 1 signaling and the other part is included in the MACsubheader of a MAC PDU. For example, Layer-1 Destination ID correspondsto the 16 bit LSB of the Destination Layer-2 ID and the Layer-1 SourceID corresponds to the 8 bit LSB of the Source Layer-2 ID, while the DSTfield in the MAC subheader includes 8 bit MSB of the Destination Layer-2ID and the SRC field in the MAC subheader includes 16 bit MSB of theSource Layer-2 ID.

Section 9.1.1.X in 3GPP R2-1915983 specifies Sidelink Control Channel(SCCH) configurations of Sidelink (SL) SRBs used for transmitting andreceiving PC5-Radio Resource Control (PC5-RRC) messages and PC5-Smessages, wherein each SL SRB corresponds to a SCCH. PC5-S messages areupper layer messages (e.g. Direct Communication Request, DirectCommunication Accept, Direct Security Mode Command, Direct Security ModeComplete, etc.), and PC5-RRC messages are used for exchanging ASconfiguration and UE capability between peer UEs (e.g.rrcReconfigurationSidelink, rrcReconfigurationCompleteSidelink,rrcReconfigurationFailureSidelink, ueCapabilityEnquirySidelink, andueCapabilityInformationSidelink, etc.).

Basically, a UE in RRC_IDLE may select resources from a sidelinkresource pool defined in system information for sidelink transmission,while a UE in RRC_CONNECTED may send a Sidelink UE Information message(e.g. SidelinkUEInformationNR) to gNB to request dedicated sidelinkresources for sidelink transmission.

Section 5.x.9.1.6 of 3GPP R2-1915983 specifies that if a PC5-RRCconnection establishment for the destination is requested by upperlayers, a UE shall establish two SL SRBs for both PC5-S messages andPC5-RRC messages respectively, and shall consider the PC5-RRC connectionis established for a specific destination. Here, a PC5-RRC connection isa logical connection between a pair of a Source Layer-2 ID and aDestination Layer-2 ID, and one PC5-RRC connection corresponds to onePC5 unicast link. Besides, a SL SRB may be associated with a Packet DataConvergence Protocol (PDCP) entity, Radio Link Control (RLC) entity, anda logical channel. Accordingly, a SL SRB may be associated with a SourceLayer-2 ID and Destination Layer-2 ID pair. Currently, it is not clearwhether each Destination Layer-2 ID is unique. If this is the case, a SLSRB may be considered to be associated with a Destination Layer-2 ID.

Since a PC5-RRC connection corresponds to a PC5 unicast link,establishment of the RRC connection means establishment of the concernedunicast link. As described above, a UE needs to exchange PC5-S messagesin order to establish a unicast link. Therefore, a first SL SRBassociated with a default Destination Layer-2 ID for PC5-S messagetransmission needs to be established when (or if) the PC5 unicast linkor the PC5-RRC connection establishment procedure is initiated (i.e.before the PC5 unicast link is established), while the UE does not needto establish the SL SRB for PC5-RRC message transmission when the PC5unicast link establishment procedure is initiated because there is noneed for both UEs to exchange PC5-RRC messages during the unicast linkestablishment.

Besides, the peer UE will transmit a security related PC5-S message(e.g. Direct Security Mode Command message) to the UE in response toreception of the first PC5-S message (e.g. Direct Communication Request)from the UE. When receiving the sidelink transmission or MAC PDU of thesecurity related PC5-S message from the peer UE, the UE needs toestablish a second SL SRB associated with the Layer-2 ID of the peer UEfor further processing the sidelink transmission or MAC PDU and/orreplying another security related PC5-S message (e.g. Direct SecurityMode Complete message) to the peer UE. In addition, the UE may alsorelease the first SL SRB when (or if) receiving the security relatedPC5-S message from the peer UE.

In one embodiment, the UE may establish a third sidelink SRB associatedwith the Layer-2 ID of the peer UE for PC5-RRC message transmission when(or if) the PC5 unicast link or the PC5-RRC connection has beensuccessfully established. Alternatively, the third sidelink SRBassociated with the Layer-2 ID of the peer UE for PC5-RRC messagetransmission may be established when (or if) the security related PC5-Smessage (e.g. the Direct Security Mode Command message) is received fromthe peer UE. The PC5 unicast link or the PC5-RRC connection issuccessfully established when (or if) a Direct Communication Acceptmessage is received from the peer UE.

It is possible there may be two separate sidelink SRBs for PC5-S messagetransmissions, one with security protection and the other withoutsecurity protection. It is also possible one sidelink SRB is used forPC5-S message transmission with security protection and the othersidelink SRB is used for security activation. If this is the case, theUE may establish a fourth sidelink SRB associated with the Layer-2 ID ofthe peer UE for transmitting PC5-S messages that are protected when (orif) the PC5 unicast link or the PC5-RRC connection has been successfullyestablished. Alternatively, the fourth sidelink SRB may be establishedwhen (or if) the security related PC5-S message (e.g. the DirectSecurity Mode Command message) is received from the peer UE. In thiscase, the fourth SL SRB may not be used for transmitting securityrelated PC5-S message.

For establishing a SL SRB associated with the Layer-2 ID of the peer UE,the UE may transmit a sidelink UE information message to gNB to requestdedicated resources for sidelink transmissions addresses to the Layer-2ID of the peer UE. The gNB may then reply a RRC Reconfiguration messageto configure the dedicated resources. Accordingly, the UE may establishthe second SL SRB or the third sidelink SRB when receiving the RRCReconfiguration message from the gNB.

In one embodiment, establishing a sidelink SRB may include establishinga PDCP entity, a RLC entity, a logical channel associated with thesidelink SRB. Furthermore, releasing a sidelink SRB may includereleasing a PDCP entity, a RLC entity, a logical channel associated withthe sidelink SRB. Also, the logical channel of the first SL SRBassociated with a default Destination Layer-2 ID for PC5-S messagetransmission may be configured with a UM RLC entity, and each of thelogical channel of the second SL SRB associated with a Layer-2 ID of thepeer UE for PC5-S message transmission and the logical channel of thethird SL SRB associated with a Layer-2 ID of the peer UE for PC5-RRCmessage transmission may be configured with an Acknowledged Mode (AM)RLC entity.

FIG. 11 illustrates an example of the above solution according to oneexemplary embodiment.

FIG. 12 is a flow chart 1200 according to one exemplary embodiment fromthe perspective of a first UE for establishing a unicast link. In step1205, the first UE establishes a first sidelink SRB for PC5-S messagetransmission if a unicast link or a PC5-RRC connection establishment isinitiated, wherein the first sidelink SRB is associated with a defaultDestination Layer-2 ID. In step 1210, the first UE transmits a firstPC5-S message for the unicast link or the PC5-RRC connectionestablishment on the first sidelink SRB with a Source Layer-2 ID derivedfrom a Layer-2 ID of the first UE and a Destination Layer-2 ID derivedfrom the default Destination Layer-2 ID. In step 1215, the first UEreceives a second PC5-S message from a second UE, wherein the secondPC5-S message is transmitted with a Source Layer-2 ID derived from aLayer-2 ID of the second UE and a Destination Layer-2 ID derived fromthe Layer-2 ID of the first UE. In step 1220, the first UE establishes asecond sidelink SRB for PC5-S message reception and/or transmission,wherein the second sidelink SRB is associated with the Layer-2 ID of thesecond UE. In step 1225, the first UE establishes a third sidelink SRBfor PC5-RRC message transmission if the unicast link or the PC5-RRCconnection establishment has been successfully completed, wherein thethird sidelink SRB is associated with the Layer-2 ID of the second UE.

In one embodiment, the first UE could establish a fourth sidelink SRBfor protected PC5-S message transmission, wherein the fourth sidelinkSRB is associated with the Layer-2 ID of the second UE. The first UE maynot establish any sidelink SRB for PC5-RRC message transmission beforethe unicast link or the PC5-RRC connection establishment has beensuccessfully completed.

In one embodiment, the second sidelink SRB could be established inresponse to reception of the second PC5-S message by a physical layer ofthe first UE. A first Sidelink Control Channel (SCCH) configurationspecified in a RRC specification could be used for establishing thefirst sidelink SRB and the second sidelink SRB. A second SidelinkControl Channel (SCCH) configuration specified in a RRC specificationcould be used for establishing the third sidelink SRB.

In one embodiment, the default Destination Layer-2 ID could beassociated with a sidelink service for which the unicast link or thePC5-RRC connection is established. The first PC5-S message could be aDirect Communication Request. The second PC5-S message could be asecurity related message.

In one embodiment, each sidelink SRB may correspond to a SidelinkControl Channel (SCCH).

Referring back to FIGS. 3 and 4, in one exemplary embodiment of a firstUE for establishing a unicast link. The first UE 300 includes a programcode 312 stored in the memory 310. The CPU 308 could execute programcode 312 to enable the first UE (i) to establish a first sidelink SRBfor PC5-S message transmission if a unicast link or a PC5-RRC connectionestablishment is initiated, wherein the first sidelink SRB is associatedwith a default Destination Layer-2 ID, (ii) to transmit a first PC5-Smessage for the unicast link or the PC5-RRC connection establishment onthe first sidelink SRB with a Source Layer-2 ID derived from a Layer-2ID of the first UE and a Destination Layer-2 ID derived from the defaultDestination Layer-2 ID, (iii) to receive a second PC5-S message from asecond UE, wherein the second PC5-S message is transmitted with a SourceLayer-2 ID derived from a Layer-2 ID of the second UE and a DestinationLayer-2 ID derived from the Layer-2 ID of the first UE, (iv) toestablish a second sidelink SRB for PC5-S message reception and/ortransmission, wherein the second sidelink SRB is associated with theLayer-2 ID of the second UE, and (v) to establish a third sidelink SRBfor PC5-RRC message transmission if the unicast link or the PC5-RRCconnection establishment has been successfully completed, wherein thethird sidelink SRB is associated with the Layer-2 ID of the second UE.Furthermore, the CPU 308 can execute the program code 312 to perform allof the above-described actions and steps or others described herein.

FIG. 13 is a flow chart 1300 according to one exemplary embodiment fromthe perspective of a first UE for establishing a unicast link. In step1305, the first UE establishes a first sidelink SRB for PC5-S messagetransmission if a unicast link or a PC5-RRC connection establishment isinitiated, wherein the first sidelink SRB is associated with a defaultDestination Layer-2 ID. In step 1310, the first UE transmits a firstPC5-S message for the unicast link or the PC5-RRC connectionestablishment on the first sidelink SRB with a Source Layer-2 ID derivedfrom a Layer-2 ID of the first UE and a Destination Layer-2 ID derivedfrom the default Destination Layer-2 ID. In step 1315, the first UEreceives a sidelink MAC PDU which includes a second PC5-S message from asecond UE, wherein the sidelink MAC PDU is transmitted with a SourceLayer-2 ID derived from a Layer-2 ID of the second UE and a DestinationLayer-2 ID derived from the Layer-2 ID of the first UE. In step 1320,the first UE establishes a second sidelink SRB for PC5-S messagereception and/or transmission, wherein the second sidelink SRB isassociated with the Layer-2 ID of the second UE.

In one embodiment, the first UE could establish a third sidelink SRB forPC5-RRC message transmission if the second PC5-S message is receivedfrom the second UE, wherein the third sidelink SRB is associated withthe Layer-2 ID of the second UE. The first UE could also establish athird sidelink SRB for PC5-RRC message transmission if the unicast linkor the PC5-RRC connection establishment has been successfully completed,wherein the third sidelink SRB is associated with the Layer-2 ID of thesecond UE.

Referring back to FIGS. 3 and 4, in one exemplary embodiment of a firstUE for establishing a unicast link. The first UE 300 includes a programcode 312 stored in the memory 310. The CPU 308 could execute programcode 312 to enable the first UE (i) to establish a first sidelink SRBfor PC5-S message transmission if a unicast link or a PC5-RRC connectionestablishment is initiated, wherein the first sidelink SRB is associatedwith a default Destination Layer-2 ID, (ii) to transmit a first PC5-Smessage for the unicast link or the PC5-RRC connection establishment onthe first sidelink SRB with a Source Layer-2 ID derived from a Layer-2ID of the first UE and a Destination Layer-2 ID derived from the defaultDestination Layer-2 ID, (iii) to receive a sidelink MAC PDU whichincludes a second PC5-S message from a second UE, wherein the sidelinkMAC PDU is transmitted with a Source Layer-2 ID derived from a Layer-2ID of the second UE and a Destination Layer-2 ID derived from theLayer-2 ID of the first UE, and (iv) to establish a second sidelink SRBfor PC5-S message reception and/or transmission, wherein the secondsidelink SRB is associated with the Layer-2 ID of the second UE.Furthermore, the CPU 308 can execute the program code 312 to perform allof the above-described actions and steps or others described herein.

FIG. 14 is a flow chart 1400 according to one exemplary embodiment fromthe perspective of a first UE for establishing a unicast link. In step1405, the first UE establishes a first sidelink SRB for PC5-S messagetransmission if a unicast link or a PC5-RRC connection establishment isinitiated, wherein the first sidelink SRB is associated with a SourceLayer-2 ID and Destination Layer-2 ID pair of a Layer-2 ID of the firstUE and a default Destination Layer-2 ID. In step 1410, the first UEtransmits a first PC5-S message for the unicast link or the PC5-RRCconnection establishment on the first sidelink SRB with a Source Layer-2ID derived from the Layer-2 ID of the first UE and a Destination Layer-2ID derived from the default Destination Layer-2 ID. In step 1415, thefirst UE receives a sidelink MAC PDU which includes a second PC5-Smessage from a second UE, wherein the sidelink MAC PDU is transmittedwith a Source Layer-2 ID derived from a Layer-2 ID of the second UE anda Destination Layer-2 ID derived from the Layer-2 ID of the first UE. Instep 1420, the first UE establishes a second sidelink SRB for PC5-Smessage reception and/or transmission, wherein the second sidelink SRBis associated with a Source Layer-2 ID and Destination Layer-2 ID pairof the Layer-2 ID of the first UE and the Layer-2 ID of the second UE.

In one embodiment, the first UE could establish a third sidelink SRB forPC5-RRC message transmission if the second PC5-S message is receivedfrom the second UE, wherein the third sidelink SRB is associated with aSource Layer-2 ID and Destination Layer-2 ID pair of the Layer-2 ID ofthe first UE and the Layer-2 ID of the second UE. The first UE couldalso establish a third sidelink SRB for PC5-RRC message transmission ifthe unicast link or the PC5-RRC connection establishment has beensuccessfully completed, wherein the third sidelink SRB is associatedwith a Source Layer-2 ID and Destination Layer-2 ID pair of the Layer-2ID of the first UE and the Layer-2 ID of the second UE.

Referring back to FIGS. 3 and 4, in one exemplary embodiment of a firstUE for establishing a unicast link. The first UE 300 includes a programcode 312 stored in the memory 310. The CPU 308 could execute programcode 312 to enable the first UE (i) to establish a first sidelink SRBfor PC5-S message transmission if a unicast link or a PC5-RRC connectionestablishment is initiated, wherein the first sidelink SRB is associatedwith a Source Layer-2 ID and Destination Layer-2 ID pair of a Layer-2 IDof the first UE and a default Destination Layer-2 ID, (ii) to transmit afirst PC5-S message for the unicast link or the PC5-RRC connectionestablishment on the first sidelink SRB with a Source Layer-2 ID derivedfrom the Layer-2 ID of the first UE and a Destination Layer-2 ID derivedfrom the default Destination Layer-2 ID, (iii) to receive a sidelink MACPDU which includes a second PC5-S message from a second UE, wherein thesidelink MAC PDU is transmitted with a Source Layer-2 ID derived from aLayer-2 ID of the second UE and a Destination Layer-2 ID derived fromthe Layer-2 ID of the first UE, and (iv) to establish a second sidelinkSRB for PC5-S message reception and/or transmission, wherein the secondsidelink SRB is associated with a Source Layer-2 ID and DestinationLayer-2 ID pair of the Layer-2 ID of the first UE and the Layer-2 ID ofthe second UE. Furthermore, the CPU 308 can execute the program code 312to perform all of the above-described actions and steps or othersdescribed herein.

In the context of embodiments illustrated in FIGS. 13-14 and discussedabove, in one embodiment, the first UE may not establish any sidelinkSRB for PC5-RRC message transmission if the unicast link or the PC5-RRCconnection establishment is initiated. Furthermore, the first UE couldrelease the first sidelink SRB if the sidelink MAC PDU including thesecond PC5-S message is received from the second UE.

In one embodiment, a first SCCH configuration specified in a RRCspecification could be used for establishing the first sidelink SRB andthe second sidelink SRB. Furthermore, a second SCCH configurationspecified in a RRC specification could be used for establishing thethird sidelink SRB.

In one embodiment, the default Destination Layer-2 ID could beassociated with a sidelink service for which the unicast link or thePC5-RRC connection is established. The first PC5-S message could be aDirect Communication Request. The second PC5-S message could be asecurity related message (e.g. Direct Auth and Key Establish or DirectSecurity Mode Command).

In one embodiment, each sidelink SRB may correspond to a SidelinkControl Channel (SCCH).

Various aspects of the disclosure have been described above. It shouldbe apparent that the teachings herein could be embodied in a widevariety of forms and that any specific structure, function, or bothbeing disclosed herein is merely representative. Based on the teachingsherein one skilled in the art should appreciate that an aspect disclosedherein could be implemented independently of any other aspects and thattwo or more of these aspects could be combined in various ways. Forexample, an apparatus could be implemented or a method could bepracticed using any number of the aspects set forth herein. In addition,such an apparatus could be implemented or such a method could bepracticed using other structure, functionality, or structure andfunctionality in addition to or other than one or more of the aspectsset forth herein. As an example of some of the above concepts, in someaspects concurrent channels could be established based on pulserepetition frequencies. In some aspects concurrent channels could beestablished based on pulse position or offsets. In some aspectsconcurrent channels could be established based on time hoppingsequences. In some aspects concurrent channels could be establishedbased on pulse repetition frequencies, pulse positions or offsets, andtime hopping sequences.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, processors, means, circuits, and algorithmsteps described in connection with the aspects disclosed herein may beimplemented as electronic hardware (e.g., a digital implementation, ananalog implementation, or a combination of the two, which may bedesigned using source coding or some other technique), various forms ofprogram or design code incorporating instructions (which may be referredto herein, for convenience, as “software” or a “software module”), orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,circuits, and steps have been described above generally in terms oftheir functionality. Whether such functionality is implemented ashardware or software depends upon the particular application and designconstraints imposed on the overall system. Skilled artisans mayimplement the described functionality in varying ways for eachparticular application, but such implementation decisions should not beinterpreted as causing a departure from the scope of the presentdisclosure.

In addition, the various illustrative logical blocks, modules, andcircuits described in connection with the aspects disclosed herein maybe implemented within or performed by an integrated circuit (“IC”), anaccess terminal, or an access point. The IC may comprise a generalpurpose processor, a digital signal processor (DSP), an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA) or other programmable logic device, discrete gate or transistorlogic, discrete hardware components, electrical components, opticalcomponents, mechanical components, or any combination thereof designedto perform the functions described herein, and may execute codes orinstructions that reside within the IC, outside of the IC, or both. Ageneral purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

It is understood that any specific order or hierarchy of steps in anydisclosed process is an example of a sample approach. Based upon designpreferences, it is understood that the specific order or hierarchy ofsteps in the processes may be rearranged while remaining within thescope of the present disclosure. The accompanying method claims presentelements of the various steps in a sample order, and are not meant to belimited to the specific order or hierarchy presented.

The steps of a method or algorithm described in connection with theaspects disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module (e.g., including executable instructions and relateddata) and other data may reside in a data memory such as RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a harddisk, a removable disk, a CD-ROM, or any other form of computer-readablestorage medium known in the art. A sample storage medium may be coupledto a machine such as, for example, a computer/processor (which may bereferred to herein, for convenience, as a “processor”) such theprocessor can read information (e.g., code) from and write informationto the storage medium. A sample storage medium may be integral to theprocessor. The processor and the storage medium may reside in an ASIC.The ASIC may reside in user equipment. In the alternative, the processorand the storage medium may reside as discrete components in userequipment. Moreover, in some aspects any suitable computer-programproduct may comprise a computer-readable medium comprising codesrelating to one or more of the aspects of the disclosure. In someaspects a computer program product may comprise packaging materials.

While the invention has been described in connection with variousaspects, it will be understood that the invention is capable of furthermodifications. This application is intended to cover any variations,uses or adaptation of the invention following, in general, theprinciples of the invention, and including such departures from thepresent disclosure as come within the known and customary practicewithin the art to which the invention pertains.

What is claimed is:
 1. A method for unicast link establishment by afirst User Equipment (UE), comprising: establishing a first sidelinkSignaling Radio Bearer (SRB) for PC5-S message transmission, wherein thefirst sidelink SRB is associated with a default Destination Layer-2Identity (ID); transmitting a first PC5-S message on the first sidelinkSRB with a Source Layer-2 ID derived from a Layer-2 ID of the first UEand a Destination Layer-2 ID derived from the default DestinationLayer-2 ID; receiving a second PC5-S message from a second UE, whereinthe second PC5-S message is transmitted with a Source Layer-2 ID derivedfrom a Layer-2 ID of the second UE and a Destination Layer-2 ID derivedfrom the Layer-2 ID of the first UE; and establishing a second sidelinkSRB for PC5-S message reception and/or transmission, wherein the secondsidelink SRB is associated with the Layer-2 ID of the second UE.
 2. Themethod of claim 1, further comprising establishing a third sidelink SRBfor PC5-Radio Resource Control (PC5-RRC) message transmission inresponse to a unicast link or the PC5-RRC connection establishment beingsuccessfully completed, wherein the third sidelink SRB is associatedwith the Layer-2 ID of the second UE.
 3. The method of claim 2, furthercomprising: establishing a fourth sidelink SRB for protected PC5-Smessage transmission, wherein the fourth sidelink SRB is associated withthe Layer-2 ID of the second UE.
 4. The method of claim 2, furthercomprising: not establishing any sidelink SRB for PC5-RRC messagetransmission before the unicast link or the PC5-RRC connectionestablishment has been successfully completed.
 5. The method of claim 2,wherein a second Sidelink Control Channel (SCCH) configuration specifiedin a RRC specification is used for establishing the third sidelink SRB.6. The method of claim 1, wherein the second sidelink SRB is establishedin response to reception of the second PC5-S message by a physical layerof the first UE.
 7. The method of claim 1, wherein a first SidelinkControl Channel (SCCH) configuration specified in a Radio ResourceControl (RRC) specification is used for establishing the first sidelinkSRB and the second sidelink SRB.
 8. The method of claim 1, wherein thedefault Destination Layer-2 ID is associated with a sidelink service forwhich a unicast link or a PC5-Radio Resource Control (PC5-RRC)connection is established.
 9. The method of claim 1, wherein the firstPC5-S message is a Direct Communication Request.
 10. The method of claim1, wherein the second PC5-S message is a security related message. 11.The method of claim 1, wherein each sidelink SRB corresponds to aSidelink Control Channel (SCCH).
 12. A first UE (User Equipment) forestablishing unicast link, comprising: a processor; and a memoryoperatively coupled to the processor, wherein the processor isconfigured to execute a program code to: establish a first sidelinkSignaling Radio Bearer (SRB) for PC5-S message transmission in responseto a unicast link or a PCS-Radio Resource Control (PC5-RRC) connectionestablishment being initiated, wherein the first sidelink SRB isassociated with a default Destination Layer-2 Identity (ID); transmit afirst PC5-S message for the unicast link or the PC5-RRC connectionestablishment on the first sidelink SRB with a Source Layer-2 ID derivedfrom a Layer-2 ID of the first UE and a Destination Layer-2 ID derivedfrom the default Destination Layer-2 ID; receive a second PC5-S messagefrom a second UE, wherein the second PC5-S message is transmitted with aSource Layer-2 ID derived from a Layer-2 ID of the second UE and aDestination Layer-2 ID derived from the Layer-2 ID of the first UE; andestablish a second sidelink SRB for PC5-S message reception and/ortransmission, wherein the second sidelink SRB is associated with theLayer-2 ID of the second UE.
 13. The first UE of claim 12, wherein theprocessor is further configured to execute a program code to: establisha third sidelink SRB for PC5-RRC message transmission in response to theunicast link or the PC5-RRC connection establishment being successfullycompleted, wherein the third sidelink SRB is associated with the Layer-2ID of the second UE.
 14. The first UE of claim 13, wherein the processoris further configured to execute a program code to: establish a fourthsidelink SRB for protected PC5-S message transmission, wherein thefourth sidelink SRB is associated with the Layer-2 ID of the second UE.15. The first UE of claim 12, wherein the processor is furtherconfigured to execute a program code to: not establish any sidelink SRBfor PC5-RRC message transmission before the unicast link or the PC5-RRCconnection establishment has been successfully completed.
 16. The firstUE of claim 12, wherein the second sidelink SRB is established inresponse to reception of the second PC5-S message by a physical layer ofthe first UE.
 17. The first UE of claim 12, wherein a first SidelinkControl Channel (SCCH) configuration specified in a RRC specification isused for establishing the first sidelink SRB and the second sidelinkSRB.
 18. The first UE of claim 13, wherein a second Sidelink ControlChannel (SCCH) configuration specified in a RRC specification is usedfor establishing the third sidelink SRB.
 19. The first UE of claim 12,wherein the first PC5-S message is a Direct Communication Request. 20.The first UE of claim 12, wherein the second PC5-S message is a securityrelated message.